Continuously shape-morphing structures have previously mostly focused on traditional kinematics with flexural components that match or exceed the deformation length scales, and/or rely on high density and high cost materials such as piezoelectric ceramics, shape memory alloys, and electro-active polymers. This has limited the size, degrees of freedom, and manufacturability of shape-morphing structures to date.
Conventionally designed and engineered fabrication methods employ digital computation and communication algorithms to control analog mechanical equipment that additively or subtractively forms shapes from masses of bulk material. Digital material systems instead propose a method for fabrication from discrete parts with discrete relative local positioning, instead of continuous variation of composition and location of material, as in an analog fabrication system. This may be thought of as printing, noting that an important distinction between digital material printing and conventional commercially available three dimensional printing processes is that digital material printing is reversible, and the information regarding the shape, assembly, and function of a finished product is intrinsic to the material that it is composed of.
Structure design and construction requires consideration of multiple factors. The design and fabrication process will generally include considerations of: 1) design requirements, 2) likely failure modes, 3) stress analysis for failure modes identified, 4) material selection and behavior, 5) fabrication, and 6) testing, all within the context of the overall design goals. For example, in order to achieve reduction in weight, increase in strength, and reduction in cost, the engineering design, materials of construction, and methods of fabrication must all be considered. In general, modern fabrication techniques include various additive and subtractive processes, employing a range of materials, including, but not limited to, composite materials, cellular materials, and digital materials.
“Composite materials” describes any two or more materials that are combined together in a single bulk material to obtain the best properties from both materials. Many industries are shifting towards the use of more composite materials because they display the single most significant consideration for any application: low weight compared to strength. The material properties of composites are unlike any material thus far, because they combine the properties of a high modulus and high tensile strength fiber for flexibility and strength, with a low modulus stiff matrix which transfers forces from one fiber to the next, creating essentially a continuous analog bulk material. Fiber-reinforced composite materials have thus enabled construction of structures having large reductions in weight for given strength and stiffness targets, but this reduction comes at the cost of very high design and processing costs and many challenges in producing mechanical interfaces (joints).
Composites are still problematic as the material of choice hindering widespread use for many reasons. First, composites vary in fibers, resins, and weaves from one manufacturer to the next, with strength and weight dependent on layup and direction of weave. Second, composites require an energy intensive process. Highly skilled technicians are never really able to have complete control over the application of pressure and heat to allow for proper curing and even distribution of heat over the entire surface [Dorworth, C. Louis, Gardiner L. Ginger, Mellema M. Greg, “Essentials of Advanced Composite Fabrication and Repair”, 2010]. Third, any flaw detected in a composite skin renders the entire material a complete waste, or makes repair difficult since creating the exact conditions to maintain bond strength is close to impossible to achieve. Fourth, not only is the composite surface designed, but the tooling and moulding for the composite is just as intensive as the final part. In the process of mitigating stress concentration, composite skins are ultimately labor intensive, time intensive, and expensive.
“Cellular materials” or “cellular solids” refers to the material structure of any living or nonliving matter, typically described as anisotropic and unidirectional or isotropic and having the same properties in all directions. Cellular materials can fill space in two-dimensions as extruded honeycomb or prismatic cells or three-dimensions as space filling polyhedra in various lattice formations. Cellular materials have been mimicked in engineered foam core structures used in construction, aerospace, and medical industries. These man made materials can be designed as highly porous scaffolds or fully dense structures which can be mechanically tuneable for a specific performance. While the science of cellular solids has enabled widespread use of lightweight materials to meet many important engineering needs, such as passive energy absorption, cellular solids are not presently in widespread use for structural applications, perhaps due to a large gap between the strength and stiffness to weight ratios of popular classical solids and the performance of known lightweight cellular materials produced from the same constituent material.
The science of cellular solids has enabled the widespread use of lightweight materials to meet important engineering needs, such as passive energy absorption, but they are not in widespread use for structural applications, perhaps due to a large gap between the strength and stiffness to weight ratios of popular classical solids, and the performance of known lightweight cellular materials that are produced from the same constituent material. The engineering of fiber reinforced composite materials has enabled structures with large reductions in weight for given strength and stiffness targets, but at very high design and processing costs, and many challenges in producing mechanical interfaces (joints).
The advances of material science in engineering of cellular solids, such as honeycomb core materials and foams, have resulted in the ability to design with lighter, more elastic, more insulating, and more energy absorptive materials. The practice of treating cellular solids as conventional continuous solids allows for simple application with conventional engineering and design methods. In the context of cellular materials, it has been noted that “constructed” periodic metal lattices allow for much larger cell size, and therefore lower relative density, compared to other methods of producing cellular metals [Wadley, H., “Cellular Metals Manufacturing”, Advanced Engineering Materials, vol. 4, no. 10, pp. 726-733, 2002].
Digital materials are comprised of a small number of types of discrete physical building blocks, which assemble to form constructions that meet the versatility and scalability of digital computation and communication systems. Digital materials have specifically been defined in prior work by Popescu as having three main properties at the highest level of description: a finite set of components or discrete parts, a finite set of discretized joints of all components in a digital material, and complete control of assembly and placement of discrete interlocking components [Popescu, G., Gershenfeld, N. and Marhale, T., “Digital Materials For Digital Printing”, International Conference on Digital Fabrication Technologies, Denver, Colo., September 2006]. Digital materials promise scalable methods of producing functional things with reconfigurable sets of discrete and compatible parts.
Digital Cellular Solids are cellular solids that exhibit improvements in relative stiffness and strength compared to relative density, over current practices for producing lightweight materials. This is accomplished by assembling lattice geometries that perform better than any currently made with traditional methods. When implemented with fiber composites, the result is not only stiffer and stronger than any previously known ultra-light material, but it presents a new scalable and flexible workflow for applying fiber composites to engineering problems.
Digital composites allow for rapid prototyping of fiber composite parts with high throughput robotic digital assemblers. The individual components may be produced through conventional means, as suited for mass production of identical parts. With digital assembly of sparse volumes composed of many smaller components, all of the tooling required may be significantly smaller than the finished assemblies. The possible properties of digital materials are myriad, and they can be designed out of any material using existing fabrication technologies and tools in order to build cellular structures for any application. Digital materials, as compared to analog materials, are completely reversible, eliminating waste by allowing individual parts to be reused and recycled at any point in the product lifecycle, no matter how large the assembly.
Architecture and civil engineering have employed space frame truss structures for many years. These have not previously been scaled volumetrically, as a perfect lattice, to the orders of units that make it practical to consider the bulk assemblies as a continuum, as would be beneficial for engineering and design purposes. Further, it is well known that space frames with many elements sharing structural duty possess highly desirable characteristics in terms of failure modes and damage tolerance [Lakes, R., “Materials with structural hierarchy”, Nature, vol. 361, pp. 511-515, 1993; Huybrechts, S., & Tsai, S. W., “Analysis and Behavior of Grid Structures”, Composites Science and Technology, vol. 56, pp. 1001-1015, 1996]. This is evident in “geodetic” airframe designs [Paul, D., Kelly, L., Venkaya, V., & Hess, T., “Evolution of U.S. Military Aircraft Structures Technology”, Journal of Aircraft, vol. 39, no. 1, pp. 18-29, 2002]. The current state of robotic manufacturing technology makes it easy to see how massively parallel assembly of digital materials can be implemented, including the assembly of structures that are larger than the assembly machinery.
The commercial aerospace industry has been moving towards aircraft designs that have fewer but larger monolithic fiber composite parts, in order to produce highly tuned and lightweight structural systems that meet extreme service, monitoring, and general environmental requirements. Conventional wisdom is that larger monolithic parts are better than an assembly of smaller parts because producing effective joints between parts is highly problematic in practice. Conventional manufacturing processes have scaled up, accordingly, which requires tools (e.g., molds for defining the shape of the part), and ovens (e.g., autoclaves for polymer matrix curing) that are large enough to influence the size of the buildings that must contain them. Some may consider that the expense involved with these manufacturing methods limits the industry altogether; there is no question that it limits prototyping capabilities. Further, the per-part investment is high enough to warrant complex repair processes as defects of small relative size arise, to say nothing of their contribution to resource intensive qualification procedures [U.S. Department of Defense, Composite Materials Handbook, “Polymer Matrix Composites Guidelines for Characterization of Structural Materials”, MIL-HDBK-17-1F 1, 2002].
Leung [Leung, A. C. H., “Actuation of kagome lattice structures”, American Institute of Aeronautics and Astronautics, April 2004] showed that the Kagome lattice structure is a desirable starting point for lattice based active structures in two dimensions. Unfortunately, there was little time given to manufacturing considerations or material properties. Hutchinson [Hutchinson, R. G., N. Wicks, A. G. Evans, N. A. Fleck, J. W. Hutchinson, “Kagome plate structures for actuation”, International Journal of Solids and Structures, 2003, vol. 40, pp. 6969-6980] continues this line of inquiry, using double-layer Kagome lattices. Theoretical bounds on performance are derived, but similarly little consideration is given to fabrication, save one mention of a transient liquid phase bonding process. Both of these approaches are bound to plate-like structures, as opposed to the space-filling lattices of this approach.
Donev [Donev, Aleksandar, Salvatore Torquato, “Energy-efficient actuation in infinite lattice structures”, Journal of Mechanics and Physics of Solids, 2003, vol. 51, pp. 1459-1475] takes a more general stance, showing that it is possible to design lattice structures that reach any uniform stress state in two or three dimensions by actuating a set of bars in coordination while doing zero work. This bar actuation paradigm is characteristic of all known lattice actuation literature (including those of the previous paragraph), which differs from the global actuation framework presented here. Despite this difference, the results are very exciting. If the actuated bars are replaced with flexural degrees of freedom, any uniform strain can be achieved at only the small energy cost of deforming the flexural elements.